1. Field of the Invention
The present invention relates to a method of manufacturing a flat display panel and the resulting flat display panel, such as a plasma display panel or a liquid crystal panel, and, more particularly, to such a panel formed by adhering together a pair of substrates having electrodes and spacers, etc. formed thereon and which are assembled with a predetermined space, or gap, therebetween and sealed together along the respective peripheries, or circumferential portions, thereof.
Such a flat display panel requires precision components of highly accurate dimensions and configurations, such that the respective, opposed surfaces of the substrates are planar throughout and provide a uniform space therebetween and such that the respective circumferential portions of the substrates are properly aligned to enable accurate coating of a sealing material therebetween for reliably sealing the two substrates together.
2. Description of the Related Art
Sealing of the spaced substrates of the flat display panel is performed by coating a selected one of the substrates adjacent the circumference thereof with an elongated and continuous band, or bead, of a sealing material consisting of glass paste, etc., and then adhering the one substrate to the other substrate with the sealing material.
Known methods of coating the sealing material onto the selected substrate include a silk screen process, using a screen mask, to apply the band of sealing material in a pattern defined by the mask and a dispensing technique wherein the sealing material is dispensed from a nozzle as the nozzle is moved about the circumferential portion of the selected substrate thereby to deposit a narrow band, or bead, of the sealing material along the circumferential portion of the selected substrate. Dispensing of the sealing material from a nozzle is recognized as a preferred coating method, since more effective for realizing precision coating accuracy while also affording high speed processing. Therefore, the technique of dispensing material from a nozzle is employed in producing a plasma display panel having a large size, e.g., greater than a diagonal measurement of 30 inches, such as is utilized in a wall-mounted type television receiver.
FIGS. 7(a) and 7(b) are a plan view and a cross-sectional view, respectively, of a plasma display panel 30 comprising a lower glass substrate 31 and an upper glass substrate 32 which have been sealed together about their respective peripheries, or circumferential portions, by a band of sealing material 33 applied by the nozzle dispensing technique. The plasma display panel 30 is assembled and sealed subsequently to completion of prior steps, such as the formation of respective sets of electrodes on the opposed planar surfaces of the two glass substrates 31, 32. More particularly, a glass paste is applied as the band of sealing material 33 along a route, or path, adjacent to the circumferential portion of a selected glass substrate 31 and then the two glass substrates 31, 32 are adhered together using the bead of glass paste sealing material 31.
In the case of applying the glass paste sealing material by the nozzle dispensing technique, the glass paste is stored in a cylinder and dispensed via a nozzle onto the glass substrate. In the related art, as shown in FIG. 7(a), the coating process, and the band of coating material deposited thereby, typically begins at a point C adjacent the circumference of the glass substrate 31 and proceeds about the circumference in a continuous path along the route indicated by the arrows, to an ending point beyond the starting point C, and thus overlapping a beginning portion of the band of sealing material 33.
The coated glass paste bead then is solidified through a baking process--i.e., a heat treatment is performed under the condition that the two sheets of glass substrates 31, 32 are stacked in assembled relationship. The two glass substrates 31, 32 thus are adhered together and the gap therebetween is sealed by fusing a bead of the sealing material 33 thereto, and which has been solidified by the heat treatment.
The space between the two sheets of glass substrates 31, 32 then is evacuated to a specified vacuum condition by exhausting the air out of a through hole formed at a predetermined position in one of the glass substrates 31, 32; moreover, a selected discharge gas is injected into the gap, or discharge space, between the glass substrates 31, 32 to complete the plasma display panel, as shown in FIG. 7(b).
To facilitate a clear understanding of the foregoing, the coating of the band of glass paste has been explained with reference to FIG. 7(a), which illustrates the application of the band to a circumferential portion of a selected one of the glass substrates 31, 32, while FIG. 7(b) illustrates the two spaced glass substrates 31, 32 in the assembled and adhered-together condition. In the assembled relationship shown in FIG. 7(b), the glass paste sealing material 33 is subjected to a heat treatment to complete the sealing; the baking, however, changes the thickness of the sealing material 33 relative to the initial size of the gap between the substrates 31, 32 at the time of coating the band of sealing material thereon and thus prior to the baking step.
According to the method of coating the sealing material explained above, the band of sealing material 33 is coated along a route which has the shape of a frame, i.e., a generally rectangular path, indicated by the arrow marks shown in FIG. 7(a). The ending portion of the route overlaps a starting portion of the route, extending from the starting position C; thus, an ending portion of the band overlaps the starting portion of the band of sealing material along a common portion of the route. As a result, the band of sealing material 33 is thicker in the overlapped portion than in the remaining portions.
Moreover, in the injection process and when initially starting the application, or coating, of the band of sealing material 33 onto the glass substrate along the specified route, the amount of sealing material dispensed from the nozzle varies, or fluctuates; therefore, controlling the initial amount of sealing material at, and in the vicinity of, the starting point C is very difficult.
FIGS. 8(a) to 8(c) are cross-sectional views showing different conditions, and corresponding amounts, of the sealing material while in a holding condition at an end of a dispensing nozzle 39. As the holding condition of the sealing material 33' in the nozzle 39, the sealing material 33' may be displaced interiorally of the nozzle 39, i.e., withdrawn from the end thereof, as shown in FIG. 8(b) or the sealing material 33' may project from the end of the nozzle 39, as shown in FIG. 8(c), relative to a normal holding condition of FIG. 8(a) in which the sealing material 33' is held at a position effectively flush with the end of the nozzle 39.
Coating of the band of sealing material 33' from the nozzle 39 onto the substrate surface is achieved by applying an increased pressure to a supply of the sealing material 33' stored in the dispenser; when the coating operation is suspended, the application of increased pressure is stopped and a drawing (i.e., decreased) pressure is applied to prevent drooping of the sealing material by gravity. However, the holding conditions may differ, as shown in FIGS. 8(a) to 8(c), depending on a change of viscosity of the sealing material 33' or a small amount of leakage of the sealing material 33'.
Under the normal condition of FIG. 8(a), coating of the band of sealing material 33' can be started under a predetermined condition; however, if the sealing material 33' is withdrawn into the inside of the nozzle as shown in FIG. 8(b), the start of coating is delayed and thus the actual coating start position on the substrate surface deviates from the predetermined position (e.g., the starting position C in FIG. 7(a)). Alternatively, if the sealing material 33' projects from the end part of the nozzle as shown in FIG. 8(c), an excessive amount of material is deposited at the coating start position.
As explained above, the coating start position and the condition of the sealing material 33' may vary and thus be unstable; further, the band extending along the route from the coating start position may be overlapped and crossed by a portion of the band which progresses to the coating end position along the same route, resulting in a non-uniform thickness of the coating material in the overlapped crossed portions.
Since the space between the two glass substrates is as small as several tens of .mu.m and may range up to several hundreds of .mu.m, if the thickness of the applied coating material fluctuates, or varies, even if by only a small amount, reliable sealing between the opposing planar surfaces of the two substrates cannot be achieved. Namely, if the coating thickness is reduced at one localized position, it will permit leakage of the discharge gas supplied in the space, or gap, between the substrates; conversely, if the coating thickness is increased, it will cause a crack or a break of one of the glass substrates.
It is therefore an object of the present invention to provide a flat display panel which is free of the above problems and, furthermore, never permits leakage of the discharge gas or the liquid crystal material received in the gap between the substrates and is not susceptible to crackage or breakage of the seal or of the substrates after the sealing process is completed.